Voice over Wireless LANs

The ability to supporting voice communications such as telephone traffic over a wireless LAN can provide significant benefits. Learn why and how you should go about implementing voice over wireless LANs.

When most people think of wireless LANs (WLANs), they generally only consider
transferring data while using applications such as a Web browser, e-mail client,
for file transfer, etc. It's possible, however, to use a WLAN as the transport
system for carrying telephone traffic from mobile users as well.

Applications and Benefits

A significant benefit of mixing telephone traffic with data on a WLAN is to
provide mobility and make use of a common infrastructure. The support of a common
system for both data and voice traffic is generally simpler and less expensive
than two separate entities.

For example, many warehouses utilize two-way radios operating in licensed frequencies
to provide communications among clerks and supervisors. Mobile communication
is needed in this environment due to nomadic users and the large size of the
facilities. In many cases, warehouses utilize WLANs for providing wireless data
collection in inventory and shipping/receiving applications. By using mobile
phones designed to interface with the WLAN, a warehouse can avoid the need for
two-way radios and the corresponding operational management
functions.

The use of WLAN phones within the offices of an enterprise is compelling as
well. In this case, a company can avoid the need to wire and rewire telephone
outlets as the company size shifts. In fact, users could utilize an 802.11 phone
to make very inexpensive long distant phone calls through the Internet.

Key Components

In addition to a WLAN backbone consisting of access points and a distribution
system, a key component for implementing voice over WLANs is a telephone
equipped with an 802.11 radio. The primary players in this market are Symbol and Spectralink, which have partnered with
private branch exchange
(PBX) vendors to sell their products. These phones, however, are somewhat pricey
at about $500 each.

A company called Telesym has software that turn a PocketPC-based
personal digital assistant (PDA) into a telephone. This can be a less expensive
alternative if you already have PDAs on site. In addition, you only need to
carry one device for multiple purposes.

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Most voice over WLAN systems also requires a gateway or access point enhanced
to handled to handle special bandwidth control requirements of voice traffic.
For example, SpectraLink's gateway allows the phones to work with Cisco's Call
Manager IP phone. The gateways will add cost to the system if you plan to
effectively support voice traffic.

Despite these costs, however, an 802.11-based phone system is generally less
expensive to install and support than a wired system. Of course you'll need
to pay specific attention to the potential problems from WLANs, such as radio
frequency (RF) interference and denial of service attacks.

Performance Requirements

Performance should be your primary consideration when installing a WLAN system
that supports voice. 802.11b is
only capable of running three uncompressed audio streams smoothly. As a result,
it's important that the system compress the audio signals before transmission
in order to increase the number of supported audio streams.

Another way to increase the number of audio streams is to utilize a higher
performing standard, such as 802.11a for
the WLAN backbone. 802.11a has the capacity to handle approximately four times
as much voice traffic as 802.11b. The problem with 802.11a, though, is that
other non-voice users of the network may only be equipped with 802.11b. As a
result, consider installing access points that include both 802.11a (for voice
users) and 802.11b (for data users).

A WLAN can support voice if you implement the system with high performance
and quality of service (QoS) in mind. The problem is that the 802.11 standard
doesn't support QoS yet. The 802.11e
group is currently working on a QoS upgrade, but the ratification of the standard
is still a ways off. 802.11e will prioritize traffic on the network, making
data give way to voice packets.

Until 802.11e is available, WLANs need to deploy proprietary QoS mechanisms
to enable effective blending of voice and data. That's a problem when trying
to support voice traffic over a public WLAN. It's not practical for public hotspot
operators to mandate the use of a particular QoS method because of the corresponding
requirement of a common vendor radio card or specific software for each client.

Jim Geier provides independent consulting services to companies
developing and deploying wireless network solutions. He is the author of the
book, Wireless LANs and
offers workshops
on deploying wireless LANs.